A liquid crystal display device has advantages such as light weight, thinness, and low power consumption.
Therefore, liquid crystal display devices are being utilized for the display sections of television sets, computers, mobile terminals, and the like. Unlike a cathode ray tube (CRT) or a self-light-emitting type panel such as a plasma display panel (PDP), a liquid crystal panel of a liquid crystal display device does not emit light by itself. Therefore, in a transmission-type liquid crystal display device and a transmission/reflection combination type liquid crystal display device, a backlight is provided on the rear face of the liquid crystal panel, and displaying is performed with light which goes out from the backlight and travels through the liquid crystal panel.
Light which is emitted from the backlight is non-polarized light. In a transmission-type liquid crystal display device and a transmission/reflection combination type liquid crystal display device, polarizers are provided so as to sandwich the liquid crystal layer. The polarizers transmit a polarization component having a polarization direction which is parallel to the transmission axis, and absorb a polarization component having a polarization direction which is orthogonal to the transmission axis. Therefore, out of the light which goes out from the backlight, it is substantially a half that is transmitted through the polarizer which is closer to the backlight, and substantially a half of the light from the backlight is not utilized.
Therefore, use of a selective reflection polarizer for increasing the efficiency of light utilization is known. A selective reflection polarizer transmits one of two polarization components whose polarization directions are orthogonal to each other, and reflects the other. A selective reflection polarizer is disposed on an outgoing face of an illuminator having a backlight, for example. A selective reflection polarizer transmits most of the polarization component whose polarization direction is parallel to the transmission axis, but allows most of the polarization component that would be absorbed by a traditional polarizer to be reflected toward the backlight. A portion of the light having returned to the backlight is reflected at the backlight and changes its polarization state, and again exits the outgoing face of the backlight toward the selective reflection polarizer. A portion of the light exiting the outgoing face of the backlight is transmitted through the selective reflection polarizer. Thus, by providing a selective reflection polarizer, the efficiency of light utilization is increased, so that the luminance of the liquid crystal display device is increased 1.2 to 1.4 folds.
Selective reflection polarizers are classified into three types. A first type of selective reflection polarizer is made of a dielectric multilayer film (see, for example, Patent Document 1). This type of selective reflection polarizer has a structure (dielectric multilayer structure) such that multiple layers of a material having refractive index anisotropy within its plane and an isotropic material are stacked, and is also referred to as a dielectric multilayer film.
A second type uses a birefringent material such as liquid crystal (see Patent Document 2). This type of selective reflection polarizer is formed by orienting a mesomorphic material within the plane.
A third type includes a plurality of metal wires arrayed on a transparent substrate. The metal wires are made of a metal having a high reflectance. This type of selective reflection polarizer is also referred to as a wire grid. Through patterning of a thin metal film, the plurality of metal wires are arrayed in parallel, with a pitch which is equal to or less than the wavelength of light of interest.
These selective reflection polarizers basically have similar functions, but have their own characteristic features. In the first type of selective reflection polarizer, as in a dielectric mirror, transmission and reflection of polarized light occurs based on a refractive index difference between layers which is caused by refractive index anisotropy. For example, light is transmitted when the difference between the refractive indices of two adjoining layers in a direction which is perpendicular to the incident face is zero, whereas light is reflected at the boundary between the two layers when the difference between the refractive indices of the two adjoining layers in a direction parallel to the incident face is large, and as a result of this, selective reflection occurs. Therefore, the first type of selective reflection polarizer has a high transmittance and reflectance. However, wavelength dispersion may occur because the wavelength that is optimum for transmission and reflection is determined based on the thickness of each layer of the dielectric multilayer structure and there is a large wavelength dependence in the transmission and reflection. Therefore, in order to obtain a desired performance across the entire range of visible light, it is necessary to form a dielectric multilayer film of about 200 layers that is optimum for each of R, G, and B wavelengths of the light source of the backlight, and attach these together. In this case, fabrication is not easy, and also the attached layers will be as thick as about 150 μm. Moreover, the degree of polarization of a selective reflection polarizer is determined based on variations in the thicknesses, refractive indices and anisotropy of the respective layers, and is generally about 90%.
The second type of selective reflection polarizer utilizes a birefringent material, and therefore has a simpler structure than that of a dielectric multilayer film and is easy to produce. However, since transmission and reflection occurs in a birefringent layer, there is a large wavelength dependence and wavelength dispersion occurs, so that light from a direction which is oblique with respect to the normal direction of the principal face of the selective reflection polarizer may become tinted. As the birefringent material of the second type of selective reflection polarizer, a liquid crystal material whose orientation direction is easy to control is suitably used. In this case, a cholesteric liquid crystal is often used, whose thickness is similar to that of a liquid crystal layer of a liquid crystal panel, i.e., about to about 6 μm. The degree of polarization of this selective reflection polarizer also cannot be high, and will be similar to that of the first type of selective reflection polarizer.
With the third type of selective reflection polarizer, wavelength dispersion in the visible light region can be suppressed by setting the pitch of the metal wires to about ½ or less of 400 nm of blue, which is shorter in wavelength in the visible light region. Moreover, its characteristics are determined by the metal wires obtained through patterning a single layer of thin metal film, and it can be made thinner than the aforementioned two types, to about 0.1 μm. Furthermore, the degree of polarization of this type of polarizer depends on the spaces between the metal wires and the pitch of the metal wires; for example, if the metal wire pitch is 150 nm and the metal wire width is 75 nm, the degree of polarization is 99.9% or more, thus realizing a high degree of polarization.
In recent years, application purposes of liquid crystal display devices are being broadened, and stable operation is required even in places of high temperatures of use, e.g., an onboard display device. Moreover, for an improved image quality, improvements in the resolution and luminance of a liquid crystal display device are being required, and therefore an increased driver frequency based on a more rapid image signal processing within the liquid crystal display device and a high output power of the light sources of its backlight are required, and the temperature during use tends to increase.
On the other hand, from the standpoint of design and compactness of the liquid crystal display device, thinning of the liquid crystal display device is being required. This calls for thinner members to be used in the liquid crystal display device. For example, as a transparent substrate, a glass substrate with a thickness of about 0.2 mm, or a plastic substrate with a thickness of about 0.1 mm is used. Moreover, designing is carried out to make the optical films used in a liquid crystal display device thinner by every 10 microns.
In order to realize such a liquid crystal display device, a selective reflection polarizer which is thin and which has an excellent thermal resistance is being required. The first type needs a large number of layers to be stacked, and the second type of selective reflection polarizer is formed by sandwiching a liquid crystal layer with two films or the like, whereas the third type of selective reflection polarizer (wire grid) comprises metal wires formed on one face of a transparent substrate, and therefore can be made thin and attain a high withstand temperature.
[Patent Document 1] Japanese National Phase PCT Laid-Open Publication No. 10-511322
[Patent Document 2] Japanese Laid-Open Patent Publication No. 6-281814
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2006-47829